This invention relates to a method of and a device for manufacturing Nd-Fe-B magnet materials which exhibit a great coercive force and a large magnetic energy product, and in particular, for manufacturing an Nd-Fe-B magnet material which is used as-the rotor magnet of a high-efficiency stepping motor, etc., i.e. a columnar-ring-like magnet which is multi-pole-magnetized along the circumference thereof. Such as magnetically anisotropic magnet material is manufactured by making, through plastic working, a green compact or a densified compact thereof anisotropic which is obtained by cold-forming an magnet material powder of an alloy composed of a rare earth metal, a transition metal and boron, for example, Nd-Fe-B magnet powder prepared by the molten-metal quenching method.
As disclosed, for example, in U.S. Pat. No. 4,770,723, it has been well-known to manufacture a sintered Nd-Fe-B magnet by forming in a magnetic field, sintering and heat-treating a powder prepared by pulverizing an ingot obtained by melting together and casting Nd, Fe and B as well as some additive elements added thereto as needed. On the other hand, a fine-crystal-type Nd-Fe-B magnet is also well known in the art, which magnet is made of Nd-Fe-B alloy powder in the form of fine crystal particles (with an average grain diameter of about 0.01 to 0.5 .mu.m), so that it exhibits a high coercive force.
It is also known, as disclosed, for example, in European Patent Laid-Open Publication No. 133,758, that this fine-crystal particle type Nd-Fe-B magnet can be provided with magnetic anisotropy through upsetting.
According to other conventional methods, magnetic anisotropy can be imparted to such magnet materials through plastic working, or more specifically, through extrusion, drawing, etc. (as disclosed in Japanese Patent Laid-Open Publication No. 61-268006).
None of the above-mentioned conventional arts, however, allows such a magnet material to be used as a columnar ring-like rotor magnet of a high-efficiency stepping motor which magnet is multi-pole-magnetized along the circumference thereof. That is, in order to make such application possible, it is necessary that the area of the columnar-ring-like magnet which extends from the center to the outer peripheral section thereof is provided with a magnetic radial anisotropy or a magnetic polar anisotropy beforehand. In the sintering method, however, the coefficient of contraction of the axis which can be easily magnetized is different from that of the axis which is hard to magnetize, so that cracking or elliptical deformation of sintered compacts occurs during sintering. As a result, no products capable of being used for industrial purpose can be produced from this method. As to the upsetting method disclosed in European Patent Laid-Open Publication No. 133,758, it can only provide a uniaxial anisotropy which is parallel to the compression axis. No integral, columnar (cylindrical) ring-shaped magnet having a radial anisotropy can be produced from the method.
As for the extrusion and the drawing mentioned above, they necessitate a cutting step to obtain a final product, and there is such a problem as a part of material remains unworked, which means they are by no means to be regarded ideal from the industrial point of view.